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The predictive skill of the Australian Bureau of Meteorology's Global Assimilation and Prediction (GASP) system in the southern lower stratosphere is examined using two different sets of diagnostics: (i) conventional verification statistics used in numerical weather‐prediction studies (namely, root‐mean‐square (RMS) error, anomaly correlation, and bias), and (ii) elliptical diagnostics of the polar vortex (defined using potential vorticity on isen‐tropic surfaces). Both sets of diagnostics indicate the same variation in predictive skill for forecasts during October 1994. The stratospheric forecasts are a large improvement over persistence even at seven days, with the performance at seven days being comparable to that in the troposphere of three‐day forecasts. There is large daily variability in the forecast scores for seven‐day forecasts, and the days with below‐average scores occur when the flow (vortex) is rapidly changing. Examination of the differences in the elliptical diagnostics show that the forecast vortex is weaker, less disturbed (i.e. closer to the pole and less elongated), and rotates faster than the analysed vortex. Consistent with a weaker forecast vortex, the minimum polar temperature and maximum zonal wind are underpredicted in the forecasts. The verification statistics in the stratosphere have a large seasonal variation, although the variation is different for different statistics. The GASP RMS errors are largest (smallest) in late‐spring (summer) whereas both the ratio of GASP to persistence RMS error and the anomaly correlation indicate that the performance relative to persistence is best (worst) in late‐spring (summer).

PurposeMajor shifts in the southern hemisphere circulation occurred during the mid‐1970s concurrent with large reductions in southern Australian winter rainfall associated with decreased cyclogenesis. The purpose of this paper is to study changes in cyclogenesis over southern Australia during the last 50 years, examine the ability of coupled climate models to simulate the winter circulation changes, and discuss projected changes in winter circulation and rainfall under different climate change scenarios.Design/methodology/approachThree dimensional instability theory is used to study changes in cyclogenesis. The response of 22 coupled model intercomparison project three (CMIP3) IPCC climate models to observed increases in greenhouse gases, from pre‐industrial to the end of the twentieth century, is examined. The authors focus on two diagnostics closely related to the changes in cyclogenesis: changes in the 300 hPa zonal wind strength; and changes in the baroclinic instability. Projected changes in baroclinic instability and rainfall are investigated in SRESB1, SRESA1B and SRESA2 scenarios.FindingsThere has been a 30 per cent reduction in the growth rate of the leading storm track mode crossing southern Australia in the 1975‐1994 period, and a 37 per cent reduction in the 1997‐2006 period, when compared to 1949‐1968. Most of the CMIP3 models capture the changes in the zonal wind; only about a third the changes in baroclinic instability. Projected changes in baroclinic instability and rainfall suggest further reductions in the growth rate of storm track modes and further large reductions in rainfall over southern Australia.Originality/valueThe paper addresses a major cause of the reduction in Australian winter rainfall, and provides guidance on future‐projected changes.